I wonder if you've already answered your own question?
I'll assume you haven't, and that you're using RoboHtml:
To make a mouseover popup, go ahead and make your popup in
the usual way; In Wyswyg mode, go to Insert, Popup, and make your
choices. Click OK. This way, it will pop up if you hover
or click. Highlight the popup text. This makes it easy to
find in TrueCode.
Go into your TrueCode; you'll see someting like this: